#include<iostream>
#include<iomanip>
#include<string.h>
#include<cmath>
#include<stdio.h>
using namespace std;

int main()
{
    int n;
    while(cin>>n)
    {
      if(n>=0)  cout<<(n+2*n)*(n+1)/2<<endl;
      else cout<<(n+2*n)*(n*(-1)+1)/2<<endl;
    }
    
    //system("pause"); 
    return 0;
}
